Fix max_multiplex case on perf_event/libpfm3
Fix max_multiplex case on perf_event/libpfm3

num_mpx_cntrs was being set to 512 even though the real
maximum is 32, causing a buffer overflow and segfault.

arm-hpc/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi,arm-hpc/papi,pyrovski/papi